Cell Division


Cells divide to make new cells

Cells divide to replace old cells that have died. They divide so that we can grow. We cannot live or function without cell division. It is an important part of life.

Cells divide all of the time. In fact, every time you blink your eyes at least ten cells have divided! Cell division is also known as mitosis. Mitosis is a process that allows a cell to copy itself and divide – making two new cells.

All cells have genetic material in them. This genetic material is like a "blueprint" or "instructions" for the cell.

Genetic material tells each cell what to make. A skin cells knows that its job is to produce skin. Blood cells can only produce more blood cells. This is because each cell contains genetic material known as DNA (deoxyribonucleic acid).

In order for cells to divide they must make a complete copy of their DNA.

DNA is usually condensed inside the nucleus in a form called chromatin:

Chromatin looks like a tangled mess of string that is wound tightly to save space. If the DNA in chromatin form wasn't wound up so tightly it would not fit into the nucleus.

How does a cell then organize all of that genetic information?

Chromosomes consist of two identical pieces of information. Each side is known as a sister chromatid. Sister chromatids will split during cell division to ensure that the genetic material is split perfectly. This is one of the most important events in all of cell division.

 

Interphase

A cell spends most of its time in interphase (no division is happening).

In the center is the nucleus which holds the DNA.

 

Cell Division

Cell division happens in steps called phases.

The phases of mitosis are: Prophase, Metaphase, Anaphase and Telophase.

 

Prophase

In prophase, the nuclear envelope that holds the DNA in the nucleus breaks down. This occurs so the genetic material can be expanded and copied. The DNA will condense to form chromosomes, which are clearly visible.

Anchors called centrioles will appear to hold the spindle. The spindle is a thread-like fiber that will attach to each chromosome at its center. The spindle helps pull each side of the sister chromatid to opposite sides.

 

Metaphase

In metaphase, the chromosomes are getting ready to move to opposite sides of the cell for division. However, they must first line up so they can be divided perfectly in half.

Metaphase is really easy to spot because of the "lining up" of the chromosomes. They appear like little "X's" in the center of the cell.


Anaphase

Anaphase is marked by the ripping apart of the chromosomes. They will begin to migrate as they are pulled by the spindle to the centrioles.

You no longer see the "X" shape of the chromosomes, rather half of one. This means the genetic material is now split into two identical parts. Each side of the cell will have one copy.

 

Telophase

Telophase is the exact opposite of prophase. In Telophase, the nuclear membrane will once again surround the DNA as it condenses into chromatin. The centrioles and spindle will disintegrate and disappear.

The cell (animal) will now be nearly ready for division. It is now clear that there are almost two cells. Each one will be an exact copy of the other.

 

Cytokinesis

Cytokinesis is the last step to complete cell division.

In an animal cell, cytokinesis occurs when the two membranes of the cell "pinch off" and form two new distinct cells.
In plant cells, a cell plate forms between the two cells. A cell plate is necessary because plants have a cell wall and not a cell membrane.